This invention relates to a method for the preparation of propylene polymer-containing solid titanium trichloride suitable for use as catalyst for the polymerization of propylene and to the preparation of propylene polymers using the propylene polymer-containing solid titanium trichloride so prepared.
A process for preparing polypropylene having high bulk density has long been sought in the art because production of a high density polymer allows elevation of the slurry concentration in the reactor, resulting in an appreciable increase in production capacity. Also, a high concentration of crystalline or isotactic polymer in the product, or in other words, improvement in the stereoregularity of the polymer, leads to certain industrial advantages, such as raising the yield based on raw propylene and decreasing the quantity of non-crystalline polymers dissolved in the diluent, thus allowing a significant simplification or elimination of the process steps required to remove the non-crystalline polymer.
In recent years various methods have been proposed for polymerization of propylene with high catalytic efficiency by use of a high-activity catalyst. Nevertheless, none of these prior art methods has been able to produce the desired polymers having satisfactory bulk density and crystallinity in a high yield, and hence, the solution of this problem is of a paramount industrial significance.
One specific method which has been proposed for improving bulk density and stereoregularity of the polymeric product utilizes a catalyst system containing both titanium trichloride and an organoaluminum compound, which mixed catalyst is subjected to a preliminary treatment at a temperature lower than 60.degree. C. in the presence of propylene (Japanese Patent Publication No. 14865/74). However, this method was merely able to reduce, to some extent, the velocity of decline of bulk density and stereoregularity of the produced polymer which otherwise decline sharply with rise of the polymerization temperature during the high-temperature polymerization after preliminary treatment for obtaining a high catalytic efficiency, and hence this method still fails to meet all industrial requirements. Unfortunately, the improved effect attributed to this method was negligible where the polymerization temperature is within the range of 60.degree. to 70.degree. C. which is the most practical range in actual industrial applications. It is also essential in this method that a suitable quantity of hydrogen be present to prevent "fish-eyes" (gel component) in the molded products produced from the polymer. Maintaining the optimum hydrogen concentration in the polymerization system, however, is often very difficult in actual practice. An excessively low hydrogen loading encourages formation of "fish-eye" in the moldings while an excessively high hydrogen loading results in poor yield of the crystalline polymer. There is also the problem that even if hydrogen and propylene are both fed in predetermined quantities, the hydrogen concentration might vary due to absorption of propylene into solution.
There has been also proposed a method of activating catalysts for propylene polymerization by a technique in which propylene is absorbed gradually at a very low rate into a solution obtained by dispersing, in an inert solvent, alkylaluminum chloride and a .delta.-type titanium trichloride composition obtained by treating TiCl.sub.3.1/3AlCl.sub.3 or .beta.-type titanium trichloride containing AlCl.sub.3 with a complexing agent and then pulverizing it. (Japanese Patent Laid-Open No. 108693/76). This method, however, does not allow a high propylene feed rate and, accordingly, a long time is required for the propylene absorption treatment.